Chapter 8, Problem 8.67P

An aqueous slurry at 30°C containing 20.0 wt% solids is fed to an evaporator in which enough w ater is vaporized at 1 atm to produce a product slurry containing 35.0 wt% solids. Heat is supplied to the evaporator by feeding saturated steam at 2.6 bar absolute into a coil immersed in the liquid. The steam condenses in the coil, and the slurry boils at the normal boiling point of pure water. The heat capacity of the solids may be taken to be half that of liquid water.

1. Calculate the required steam feed rate (kg/h) for a slurry feed rate of 1.00 × 10³ kg/h.
2. Vapor recompression is often used in the operation of an evaporator. Suppose that the vapor (steam) generated in the evaporator described above is compressed to 2.6 bar and simultaneously heated to the saturation temperature at 2.6 bar, so that no condensation occurs. The compressed steam and additional saturated steam at 2.6 bar arc then fed to the evaporator coil, in which isobaric condensation occurs. How much additional steam is required?
3. What more would you need to know to determine whether or not vapor recompression is economically advantageous in this process?